Immunoglobulin class switching is the mechanism through which a given antibody combining site can be expressed on antibodies that mediate distinct biologic functions. An assay has been developed that allows the quantitative measurement of the degree of switch region recombination. This technique, digestion-circularization polymerase chain reaction (DC- PCR) was used to measure the frequency of class switching in vitro in cells stimulated with lipopolysaccharide and IL-4. It was shown that sufficient numbers of switched chromosomes existed to explain virtually all the Ig class switching implying that other methods such as transplicing or alternative splicing of full length transcripts were not required to explain the class switching even in the earliest phases of the process. The mapping of the gene for the murine x-linked immunodeficiency gene (xid), which controls B cell responses to polysaccharide antigens was begun. The approach used was high resolution mapping utilizing probes specific for polymorphisms to type the alternative forms of PCR products from an extended set of loci flanking xid. Once the gene is located precisely, "walking" through a Yac-library will be undertaken with the goal of identifying the xid gene itself and gaining functional information regarding the gene.